Submerged repeater supervision is the basic reason for special terminals in submarine lightguide transmission systems. However, repeater supervision is also the major hurdle in creating a submerged multiplex for such systems.
Currently line monitoring and fault locating functions are used fro maintaining digital transmission systems. One method used for line monitoring is the determining of a bit error rate by inserting at the transmitting terminal a block parity bit in a predetermined position in the data stream after each block of data. Each block of data and its block parity bit are analyzed for block parity violations at the receiving terminal or at an intermediate point along the line. Any block parity violations are counted over a period of time and are computed into a bit error rate.
Fault locating is a procedure used for determining which repeater section of the line is the source of a trouble condition when the line monitoring arrangement indicates there is an inordinately high bit error rate.
One useful control signalling arrangement for a pulse transmission system, as described in U.S. Pat. No. 4,633,464, uses a main data stream including a block parity channel. Control signals are transmitted from an end terminal to repeaters by inserting the control signals, as inverted, or complemented, parity bits, in periodically spaced bit positions of the block parity channel. At the repeaters, the control signals are separated from the block parity signals and are decoded for controlling selectively circuits or devices for maintenance operations. For that pulse transmission system, a maintenance response signalling arrangement, as described in U.S. Pat. No. 4,586,186, uses the main data stream for transmission of maintenance response signals between the repeaters and the end terminal. From any repeater, response information is modulated onto a subcarrier which is superimposed as a jitter signal onto the main data stream. At the end terminal, the response information is recovered from the jitter signal on the main data stream.
The supervision of undersea repeaters is based on a customized signal format provided by one of the terminals. This format includes a block parity bit for every block of twenty-four data bits. These block parity bits serve two functions. First of all, parity errors can be determined at the repeaters for monitoring the block error performance of the line. Secondly by intentionally inverting periodically one of the parity bits, the terminal sends command signals to the repeater and the repeater generates a subcarrier for sending responses back to the commanding terminal. Preserving these two functions presents major difficulties in using previously existing digital multiplex achitectures in undersea systems.
Typical digital multiplex architectures either interleave plural bit streams by bit interleaving or by time division multiple access. Any multiplex which performs bit interleaving destroys any block parity function of the tributaries because the bit count of the main data stream is the total of the bit counts of the plural tributary bit streams. The block parity bits of the tributary lines become meaningless in the multiplexed data stream. Alternatively, a time division multiple access arrangement, receives signals from plural tributaries operating at the same data rate as the multiplexed line. The signals on the tributaries are bursty in the sense that there are gaps between blocks transmitted on each tributary line. The gaps on one tributary line concur with the blocks of data transmitted on other tributaries. The blocks are interleaved in the multiplexed data stream, however, guard intervals are included between the blocks of data on both the tributary lines and on the multiplexed data stream. Although these guard intervals assure that the data from one tributary line do not interfere with the data from another tributary line, the guard intervals reduce the efficiency of the line. That is the guard intervals are unused transmission capacity.
Order wire operation from a station at one terminal to a station at another terminal also is based on the customized signal format provided by the terminal. This format periodically provides bits specifically designated for communications originating in a station located at one terminal and terminating in a station located at a second terminal. Transmitting these order wire signals also presents a problem for previously existing multiplex architectures.